1. Field of the Invention
This invention relates to the active control and reduction of both tonal and broad band noise in rotating machinery, and in particular to the reduction of tones and noise in air moving devices.
2. Description of Related Art
Rotating equipment emits sounds which are often objectionable to humans or which induce further vibrations in other equipment. The noise is discernible over a broad frequency spectrum, often with significant contributions from the lower end of the frequency spectrum--that below 1000 HZ. Within a broad frequency band of sounds there is a general noise level which may be due to turbulent flow and also high amplitude discrete tones which correspond to the frequency of the repetitive motion of parts of the machinery. The tones are caused by slight imbalances in machine parts, by air moving device blades moving past a stationary object, or by the excitation of natural modes of vibration within each element of the machinery.
Passive control of machine noise includes the use of enclosures which are lined with materials that absorb the offensive acoustic energy. Some machinery, however, requires access for convection cooling of the machinery itself, or openings for the output of the machinery as in the case of air moving devices, compressors, or turbines. Therefore, active methods of control have been devised which sense objectionable sound emanating from rotating machinery, generate additional sound which is out of phase with the detected sound, and thereby lowers or cancels it.
Active control methods typically sense structure-borne vibration or air-borne acoustic noise, or both, operate upon these signal(s), and generate additional sound which is separate from the source of the objectionable noise. For the case of flow induced noise, the radiated noise is related to lift fluctuations caused by the flow. These lift fluctuations can be sensed and used as inputs to a control algorithm.
An example is U.S. Pat. No. 5,117,642 (K. Nakanishi, et. al.) which shows a compressor within a chamber with an opening whose longest lateral dimension is small compared to the wavelength in air of the objectionable noise, a vibration sensor which feeds a control circuit containing a finite impulse response filter, and a sound generator which is mounted close to the opening which delivers acoustic energy into the air. The attempt here is to cancel the offensive sound before it can radiate from the chamber. The same inventors further disclose detecting vibration in a direction tangential to the compressor in U.S. Pat. No. 5,127,235.
U.S. Pat. 5,010,576 (P. D. Hill) teaches the use of an accelerometer which detects imbalances on a multiblade air moving device, a speaker which is mounted facing the air moving device and coaxial with its hub, a microphone which detects the sounds from both the air moving device and the speaker, and the use of a least mean square adaptive filter which accommodates for a time differential in the sounds reaching the microphone. Cancellation of the objectionable sound is made by generation of an out-of-phase acoustic signal generated by a nearby loudspeaker.
U.S. Pat. No. 4,837,834 (M. C. Allie) discloses the acoustic attenuation of noise in ducts whereby one microphone senses noise at an upstream point in the duct, a speaker introduces canceling sound into a mid portion of the duct, and another error sensing microphone senses the resultant acoustic field at a downstream position. The invention is primarily directed to signal filtering, processing, and modeling to drive loudspeakers which cancel sounds in the air.
U.S. Pat. No. 4,817,422 (R. M. Allen) shows an aeroacoustic wind tunnel test apparatus wherein one or more acoustic coupling means inject sound into the upstream end of a flow passage at predetermined intervals. A loudspeaker driver transmits sound along the longitudinal axis of the apparatus.
In the electronic arts there is an ongoing exponential increase in the number of electronic components per unit volume of space. This trend accentuates the need to remove heat which is generated by each component. Air moving devices which provide forced air cooling within component enclosures are also required to decrease in size or else the ratio of packaging volume to active component volume becomes unacceptably large. Less space is also available for passive sound filters and absorbers. Similarly, small cooling air moving devices running at high speeds are used resulting in high tonal and broad band noise levels.
Accordingly, there is an increased need for active intervention to detect and cancel both tones and broad band noise on rotating machinery, and particularly in air moving devices for cooling electronic equipment.